Field of the Invention
The present invention relates to a direct current to direct current converter and an inverter system, and especially relates to a bi-directional direct current to direct current converter and a grid-connected inverter system.
Description of the Related Art
FIG. 1 shows a block diagram of a related art grid-connected inverter system. A related art grid-connected inverter system 70 comprises a direct current voltage supply unit Vin, a bi-directional direct current to direct current converter 10, an inverter 20, a power grid 50 and a relay 60. The direct current voltage supply unit Vin comprises a voltage positive side VinP and a voltage negative side VinN. The bi-directional direct current to direct current converter 10 comprises a first inductor 102, a first switch 104, a first capacitor 106, a middle voltage point BusN, a second switch 110, a second capacitor 112 and a switch controller 118.
The direct current voltage supply unit Vin provides the bi-directional direct current to direct current converter 10 with an input direct current voltage 32. When the second switch 110 is turned on and the first switch 104 is turned off, the first inductor 102 stores energy. When the second switch 110 is turned off and the first switch 104 is turned on, the first inductor 102 releases energy to the first capacitor 106 and the second capacitor 112.
In another word, the switch controller 118 turns on or turns off the first switch 104 and the second switch 110, so that the bi-directional direct current to direct current converter 10 converts the input direct current voltage 32 into an output direct current voltage 34. The bi-directional direct current to direct current converter 10 sends the output direct current voltage 34 to the inverter 20. Moreover, the bi-directional direct current to direct current converter 10 sends the output direct current voltage 34 to the inverter 20 through the first capacitor 106 and the second capacitor 112.
For the inverter 20, the middle voltage point BusN is not connected to ground, but the electric potential of the middle voltage point BusN is controlled to be equal to ground. As shown in FIG. 1, in the boost architecture of the related art grid-connected inverter system 70, the voltage of the voltage negative side VinN relative to ground is |Vbus/2|, and the voltage of the voltage positive side VinP relative to ground is |the input direct current voltage 32−(Vbus/2)|, wherein the Vbus is the voltage difference between a first point p and a second point q in FIG. 1. Therefore, if the city power is AC200V, the Vbus is at least greater than the city power 350V. At this time, the voltage of the voltage negative side VinN relative to ground is 175V.
In conclusion, the voltage of the boost architecture relative to ground shown in FIG. 1 is related to the Vbus. However, some countries have laws and regulations for the input apparatus of the non-solar panel. For example, the input voltage relative to ground has to be less than 150V in the section 1300 of JEAC8001-2011 of Japan.
FIG. 2 shows a voltage simulation diagram of the related art grid-connected inverter system. As shown in FIG. 2, when the city power is AC200V, the voltage of the voltage negative side VinN relative to ground is 175V. Because the voltage of the boost architecture relative to ground shown in FIG. 1 is related to the Vbus, the voltage of the voltage negative side VinN relative to ground is not in conformity with the laws and regulations of Japan mentioned above. Therefore, an extra isolation apparatus has to be arranged to be in conformity with the laws and regulations of Japan mentioned above. The cost is increased.